1. Field of the Invention
The present invention relates to methods and apparatus for communicating over a single optical fiber communications channel, and more specifically, to methods and apparatus for converting electrical signals to optical signals and optical signals to electrical signals by a single laser diode which acts as both a light-emitting diode and a photodetection diode.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The telecommunications industry is using more and more optical or light fibers in lieu of copper wire. Optical fibers have a extremely high bandwidth, thereby allowing significantly more information to be carried than can be carried by a copper wire.
Of course, modern communication systems require bidirectional communications, where each station on a communications channel can both transmit and receive. This is true, of course, whether using electrical wiring or optical fibers as the transmission medium. Early telephone communication systems solved this need simply by providing separate copper wires for carrying the communications in each direction. Some early attempts at using optical fibers as a transmission medium followed this example and also used two different optical fibers 10 and 10A for carrying the communications in each direction as illustrated in the prior art FIG. 1. As shown, fiber 10 is connected by an optical coupler 12 to an LED (light-emitting diode) 14 at one end and by optical coupler 16 to a PD (photodetection diode) 18 at the other end. Similarly, but in reverse, fiber 10A is connected by an optical coupler 16A to PD 18 at one end and by optical coupler 12A to LED 14 at the other end.
However, because of the extremely high bandwidth available for use by an optical fiber a single fiber is quite capable of carrying communications in both directions. In fact, as will be appreciated by those skilled in the art, by using modern multiplexing techniques a single fiber has sufficient bandwidth to carry a large number of different communications by different customers in both directions at the same time. One such technique is (WDM) (wavelength divisional multiplexing) which is shown in the prior art FIG. 2 and uses different wavelengths for each direction of travel. Components in FIG. 2 and subsequent figures which operate the same as shown in FIG. 1 carry the same reference numbers. In the embodiment shown in FIG. 2, a central office 20 is connected to an RDT (remote digital terminal) 22 by a single optical fiber 10B. As shown, the central office includes a light-emitting diode 14 optically connected to fiber optics 10 by optical coupler 12 for converting electrical signals to optical signals and a photodetection diode 18 optically connected to optical fiber 10B by coupler 16A for converting optical signals to electrical signals. The fiber optics 10 and fiber optics 10B are each connected to a wavelength division multiplexor 24 which in turn is connected by optical coupler 26 to optical fiber 10B. This arrangement is duplicated at the RDT 22 by light-emitting diode 14A, photodetection diode 18A, and wavelength division multiplexor 24A. It will, of course, be appreciated that although the figure is shown as providing communications between a central office 20 (station 1) and a remote terminal office 22 (station 2), the communications system could be used for providing communications between any two types of stations such as, for example, two central offices, two remote terminal offices, or between a remote office and an individual user""s location etc. A typical communication system using an LED and a PD with a single optical fiber is disclosed in U.S. Pat. No. 5,075,791 entitled xe2x80x9cMethod and Apparatus for Achieving Two-Way Long Range Communication over an Optical Fiberxe2x80x9d, issued to Mark W. Hastings, and incorporated in its entirety hereby by reference.
Yet another technique for using a single optical fiber 10B is illustrated in the prior art FIG. 3. The illustrated figure is referred to as TCM (time compression multiplexing) and is sometimes referred to as a xe2x80x9cping-pongxe2x80x9d system. The system uses a single optical fiber 10 and a single diode 30 and 30A at each end connected by optical coupler 32 and 32A, respectively, for both converting electrical signals to optical signals and for receiving optical signals and converting those optical signals to electrical signals. TCM systems have the obvious advantage of requiring fewer components. Unfortunately, diodes presently available for such dual use while having a high efficiency of converting electrical signals to optical signal for transmission in one direction down the optical fiber, are not as efficient at receiving the very low-level optical or light signal from the fiber and converting that optical signal to an electrical signal. This is especially true after the significant attenuation the optical signal will experience as it travels down the optical fiber. Consequently, present systems using a signal diode for both transmitting and receiving are often noisy and ineffective.
Thus, it would be advantageous to provide a signal diode system which can receive the low amplitude optical signal and make the conversion to an equivalent electrical signal amplifying the electrical signal and still maintain a low signal-to-noise ratio.
The present invention addresses the above concerns and disadvantages of prior art systems for using a signal diode for both transmitting and receiving optical signals.
The circuitry of a preferred embodiment of the invention for using a signal diode connected to an optical fiber for both receiving and transmitting digital signals provided by a drive unit connected to the diode and making up a part of the circuitry of this invention is described. The drive unit receives electrical digital input signals such as PWM (pulse-width modulated) signals from a suitable digital source and then stabilizes parameters of these signals such as the voltage level and the duty cycle of the PWM signal before providing the signal to a current amplification device which then provides the amplified current signal to drive the diode. A preferred embodiment of the invention further includes a temperature compensation circuit for maintaining the output level of the drive circuit within acceptable limits even with temperature variations. The optical fiber connected to the circuitry is, of course, suitable for transmitting digital light signals in both directions. The diode is optically coupled to the optical fiber and is controlled by the inventive circuitry so as to have a receiving mode and a transmitting mode. In the receiving mode, the diode receives the digital light signals from the optical fiber and generates electrical digital signals representative of the received digital light signals. In the transmitting mode, the diode is electrically connected to the electrical digital signal source and receives the electrical digital signals and generates optical or light digital signals representative of the electrical digital signals for transmission by the optical fiber. A preferred diode for both receiving and transmitting is an ELED (edge-emitting light-emitting diode). The electrical signals generated by the diode in response to receiving the optical digital signals are provided to a receiver circuit which is connected to the diode electrical outputs. The receiver circuit amplifies and conditions the electrical signals from the diode in a manner such that the signals are suitable for being provided as electrical digital output signals for communication purposes. In a preferred embodiment, a receiver circuit has amplifying circuitry which includes an automatic gain control amplifier and further includes a slicing comparator for calculating the slicing level of the output signal. Since the diode includes a single set of input/output leads, it will be appreciated that the drive circuit and the receive circuit of the diode are electrically connected. Consequently, when the drive circuit is providing drive current to the diode, amplifiers in the receiving circuit may be driven to saturation. Further, it will take a finite time period for the junction of the diode to discharge after receiving the last electrical transmission pulse. Consequently, there is further included an isolation circuit and a sweep circuit for removing residual charge from the diode subsequent to the diode being in a transmission mode and prior to the diode operating in the receiving mode. According to one embodiment, such isolation is accomplished by a switching circuit which disconnects the amplifying circuit at two locations during the transmission mode.
Thus, the present invention presents a number of advantages over prior art single diode circuits.